I agree to that. It is common that those who stutter in everyday life stop stuttering when they sing or play in a theater performance for example. It is very similar in languages - when you are not 100% fluent in a foreign language you are in a way playing a role of someone who pretends who knows how to speak. Judging by my own experience - I started stuttering in another language when I became fluent. Before that - no signs of stuttering for several years. Really interesting)